A well-known method for producing amino acids is the fermentation method using microorganisms belonging to the genera Corynebacterium, Brevibacterium, Escherichia, Microbacterium, Serratia, Bacillus and Pseudomonas, and the like (non-patent publication No. 1).
With regard to the above fermentation method, various techniques have been developed for raising the production efficiency. One of these techniques is a process in which crystallization of L-amino acid in a culture is induced by adjusting the temperature and pH of a culture medium or by adding a surfactant to the culture to maintain the L-amino acid concentration in the culture below a certain level, whereby production inhibition by accumulation of L-amino acid at a high concentration can be avoided (patent publication No. 1).
Also known are a process for crystallizing L-glutamic acid (L-Glu) having a relatively low solubility in a culture medium by using a medium adjusted to pH suitable for deposition of L-Glu (patent publication No. 2) and a process for depositing α crystals of L-phenylalanine (L-Phe) in a culture by adding α crystals of L-Phe to the culture or changing the pH of the culture to a value of 7.8 to 8.3 at the stage where the L-Phe concentration in the culture is beyond the saturation solubility (patent publication No. 3).
However, as the culture contains microcrystals of amino acid in the above processes, direct separation of crystals and microbial cells based on the difference in particle size can give only a low yield of amino acid. In order to raise the yield of amino acid in these processes, it is necessary to dissolve crystals of amino acid accumulated in the culture, for example, by addition of water to the culture or heating of the culture, and after microbial cells are separated from the culture by using a centrifugal or filtration separator or the like, crystallization by concentration should be carried out.
As the method for directly separating crystals of amino acid and microbial cells, a method using a liquid cyclone is known (patent publication No. 4). However, the recovery rate of crystals of amino acid by this method does not reach 80%.
As described above, a process for producing an amino acid by fermentation in which the amino acid is crystallized in a medium during the culturing is excellent, but a more efficient process is still needed.    Non-Patent Publication No. 1:            Amino Acid Fermentation, Gakkai Shuppan Center (1986)            Patent Publication No. 1:            Japanese Published Unexamined Patent Application No. 288/87            Patent Publication No. 2:            Japanese Published Unexamined Patent Application No. 238593/02            Patent Publication No. 3:            Japanese Patent No. 3239905            Patent Publication No. 4:            Japanese Patent No. 2958789        